This application claims the priority of German Patent 199 27 846.6, filed Jun. 18, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method of monitoring an internal-combustion engine in which pressure values of the cylinder space of the internal-combustion engine are measured by means of a pressure sensor and the condition of the sensor is determined therefrom.
The proper functioning of an internal-combustion engine is monitored during the operation by an electronic engine control unit. Thus, it is known, for example, from German Patent Document DE 41 27 950 A1 to monitor the sensor detecting the internal pressure of the cylinders. In this case, the pressure values are checked as to whether they are within a tolerance band with variable limit values. The limit values, in turn, are a function of the loading of the internal-combustion engine. Pressure values outside the tolerance band are interpreted as being faulty. As a subsequent reaction, the corresponding cylinder is changed to idling. It is problematic that a short-term disturbance of the sensor signal or the aging of the sensor will necessarily cause a cylinder switch-off; that is, the internal-combustion engine will no longer operate normally.
A diagnostic process for pressure sensors of an internal-combustion engine is known from European Patent Document EP 0 569 608 A1. In this process, the pressure values in the compression cycle or in the upper dead center are compared with a reference value. In the case of a persistently faulty pressure sensor, a visual or acoustic fault indication is activated as a consequent reaction.
Based on the above-described prior art, it is an object of the invention to ensure the normal operation of the internal-combustion engine as long as possible.
This object is achieved in that pressure values of the cylinder space are continuously computed from current parameters of the internal-combustion engine by means of a mathematical model. At any point in time, a measured pressure value and a computed pressure value are present isochronously. From the comparison of the computed pressure values and the measured pressure values, information is then obtained concerning the condition of the pressure sensor. With the detection of a permanently faulty pressure sensor, the latter is deactivated. The further operation of the internal-combustion engine takes place on the basis of the computed pressure values; that is, the computed pressure values are set as the relevant pressure values.
A permanently faulty pressure sensor is detected if, in a first step, a deviation is present outside a tolerance band consisting of a first and second limit value, and in a second step, the amount of the deviation is greater than a third limit value. The deviation is determined from the measured and computed pressure values.
If it is detected in the second step that the amount of the deviation is smaller than the third limit value, the measured pressure values are corrected.
The method according to the invention therefore permits a normal operation of the internal-combustion engine even if, for example, a short-circuit of the sensor is present. It is another advantage that a faulty pressure sensor signal can be corrected by the reference values, for example, in the case of a zero-point fault or amplification fault of the pressure sensor.
The mathematical model for computing the pressure values of the cylinder space is represented by a differences equation. The description of the thermodynamic relationships by way of a differences equation has the advantage that the method is real-time-capable. This means that with respect to a measured pressure value, a correspondingly computed pressure value is present at any point in time during the operation of the internal-combustion engine.